Most chargers used in mobile phone and digital cameras adopt switch mode power supply (SMPS). Generally, these charger SMPS includes AC input and PWM control circuit, transformer and constant current/voltage control circuit, wherein the AC input, PWM control circuit, constant current and/or constant voltage control circuit are coupled through an optical coupling element. The input side of the transformer is connected to the AC input and PWM control circuit; the output side of the transformer is connected to the constant current/voltage control circuit of the charger. All these circuits adopt step-down transformer and the output side of this kind of transformer usually has only one single winding. In the prior arts described above, the transformer generally utilizes two kinds of output control circuits, one is an output control circuit constituted by the dual operational amplifier and the voltage stabilizing circuit and the other is an output control circuit constituted by the voltage stabilizing circuit to achieve constant voltage and a transistor to achieve the constant current.
FIGS. 1A, 1B and FIGS. 2A, 2B are circuit diagrams showing two kinds of charger circuits in the prior arts.
Please refer to FIG. 1A, which shows a first kind of common charger circuit 100A. The circuit 100A includes an AC input and PWM control circuit 102, a transformer T1, a constant current control circuit 104, and a constant voltage control circuit 106, wherein the constant current control circuit 104 and the constant voltage control circuit 106 are coupled with the AC input and PWM control circuit 102 through an optical coupling element U3. In the charger circuit 100A, the input side of the transformer T1 has an input coil 101a and an input auxiliary coil 101b, and the output side thereof has an output coil 101c, which first terminal is series connected to the second terminal of the coil 101c through a diode D9 and a capacitor C9, wherein the first terminal of the output coil 101c is connected to the positive electrode of the diode D9. In the circuit 100A, resistors RV1 and RV2 are for voltage stabilizing to achieve constant voltage, resistor R10 and capacitor C7 are voltage compensative elements, resistor R9 and capacitor C8 are current compensative elements, RI is to achieve constant current, and other elements including capacitor C11, resistors R8 and R17, are auxiliary electronic elements.
FIG. 1B shows a charger circuit 100B which is the extended design of the charger circuit 100A shown in FIG. 1A, in which some auxiliary elements are added. Please refer to FIG. 1B, the charger circuit 100B includes an AC input and PWM control circuit 102, a transformer T1, a constant current control circuit 104, and a constant voltage control circuit 106, wherein the constant current control circuit 104 and the constant voltage control circuit 106 are coupled with the AC input and PWM control circuit 102 through an optical coupling element U3. In the charger circuit 100B, the input side of the transformer T1 has an input coil 101a and an input auxiliary coil 101b, and the output side thereof has an output coil 101c, which first terminal is series connected to the second terminal of the coil 101c through a parallel circuit and a capacitor C9, wherein the parallel circuit includes a first branch circuit constituted by capacitor C12 and resistor R18, and a second branch circuit constituted by diode D9, and the first terminal of the output coil 101c is connected to the positive electrode of the diode D9. In the circuit 100B, resistors RV1 and RV2 are stabilizing resistors, resistor R10 and capacitor C7 are voltage compensative elements, resistor R9 and capacitor C8 are current compensative elements, RI is to achieve constant current, and other elements including capacitor C11, resistors R8 and R17, are auxiliary electronic elements. The circuit 100B further includes inductances L1 and L2 for filtering.
Referring to FIGS. 1A and 1B, the principles of the charger circuits 100A/B are similar, as followed: The output coil 101c generates output voltage rectified by the diode D9, the output voltage is divided by the resistors RV1/RV2 and connected to the voltage reference input terminal of the constant voltage control circuit 106. The constant current resistor RI senses the output voltage for providing constant current, wherein resistor R10 and capacitor C7 are voltage compensative elements, resistor R9 and capacitor C8 are current compensative elements, the power for the optical coupling element U3, the constant current control circuit 104 and the constant voltage control circuit 106 are directly supplied by the output side, that is, the power supply is obtained from the second terminal of the output coil 101c, and the inductances L1 and L2 are used for filtering and are dispensable elements. The charger circuit 100A/B are deficient in hardly controlled short circuit current and poor output characteristics.
FIGS. 2A and 2B show the second kind of charger circuit in the known technology. First, please refer to FIG. 2A, the difference between the charger circuit 200A and the circuit 100A shown in FIG. 1A is the peripheral circuits of the output coil 101c. The first terminal of the output coil 101c is connected to the second terminal of the output coil 101c through a series connected capacitor C9 and a parallel connected circuit, which includes a first branch circuit constituted by diode D9 and a second branch circuit constituted by excited circuit. In the embodiment shown in FIG. 2A, the excited circuit includes, in turn, diode D8, resistor R12 and capacitor C10, which are series connected, wherein the second terminal of the output coil 101c is connected to the negative electrode of the diode D9 and also to the positive electrode of the diode D8. As compared with the circuit 100A, in the circuit 200A, the excited circuit constituted by the diode D8, the resistor R12 and the capacitor C10 is added to supply power for the optical coupling element U3, the constant current control circuit 104, and the constant voltage control circuit 106. In the excited circuit, the positive electrode of the diode D8 is connected to the second terminal of the output coil 101c, the negative electrode of the diode D8 is connected to both the resistor R12 and the capacitor C10, which are series connected, and the other end of the capacitor C10 is connected to the capacitor C9, and further, a connecting point located between the resistor R12 and the capacitor C10 is drawn out for providing power to the optical coupling element U3, the constant current control circuit 104, and the constant voltage control circuit 106. The charger circuit 200A can improve the performance of short circuit and output characteristic, but the EMC characteristic becomes worse.
Please refer to FIG. 2B, in which the charger circuit 200B is the extended design of the charger circuit 200A shown in FIG. 2A. Comparing the circuit 200B and 100B, the difference is the peripheral circuit of the output coil 101c. The first terminal of the output coil 101c is connected to the second terminal of the output coil 101c through a series connected capacitor C9 and a parallel connected circuit, which includes a first branch circuit constituted by diode D9, a second branch circuit constituted by capacitor C12 and resistor R18, which are series connected, and a third branch circuit constituted by excited circuit. In the embodiment shown in FIG. 2B, the excited circuit includes, in turn, diode D8, resistor R12 and capacitor C10, which are series connected, wherein the second terminal of the output coil 101c is connected to the negative electrode of the diode D9 and the second terminal thereof is connected to the positive electrode of the diode D8. As compared with the circuit 100B, in the circuit 200B, the excited circuit constituted by the diode D8, the resistor R12 and the capacitor C10 is increased for being the parallel connected third branch circuit and for providing power to the optical coupling element U3, the constant current control circuit 104, and the constant voltage control circuit 106. In the excited circuit, the positive electrode of the diode D8 is connected to the second terminal of the output coil 101c, the negative electrode of the diode D8 is connected to the resistor R12 and the capacitor C10, which are series connected, and the other end of the capacitor C10 is connected between the resistor R18 and the capacitor C9, and further, a connecting point located between the resistor R12 and the capacitor C10 is drawn out for providing power to the optical coupling element U3, the constant current control circuit 104, and the constant voltage control circuit 106. Although the charger circuit 200B can solve the problems of short circuit and poor output characteristic, the EMC characteristic also becomes worse.
Therefore, it is necessary to provide a new and high performance charger circuit with improved electromagnetic compatibility characteristic, high conversion efficiency and excellent short circuit characteristic.